Considering xenForo - questions about skin and login

I am really considering going for xenForo, have read about several forum solutions and it looks real interesting, I dont have a forum yet, so it will be a fresh install.

Question 1: I see there are sites offering skins to xenForo, are there any design specs somewhere that I can give to my designer so he can design the forum to resemble my website, so that the visitors will experience both the site and forum as one design!

Question 2: Are there any important things to consider with the design of a own skin, so that we dont make the forum itself slow down or bug or anything, or asked in another way, can the fact that we make a skin affect the performance of the forum at all?

Question 3: If it is possible to wreck the forum making own skin, do you offer inhouse modification of the design? Or have anyone you can reccomend me using that has experience with xenforo skin modification?

Question 4: I am going to have my users log into my main site using either a regular email/pass login or a OAuth login, will the forum software require a own login or will I be able to set ut up so that if they log into the main site they will also be logged into the forum? And other way around if they log into the forum first they will also be logged into the main website?

Question 4: I am going to have my users log into my main site using either a regular email/pass login or a OAuth login, will the forum software require a own login or will I be able to set ut up so that if they log into the main site they will also be logged into the forum? And other way around if they log into the forum first they will also be logged into the main website?

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XenForo has it's own authentication method and user database.

You can have a custom authentication add-on created though, for an SSO system.